For example, blood pressure often falls suddenly during dialysis. For early detection of such an event, it is necessary to measure blood pressure on a frequent basis, and ideally on a continuous basis during dialysis. The most common method involves measuring blood pressure intermittently, but frequently, with a manchette of a sphygmomanometer kept attached. However, this method has problems. Namely, measurements can be taken at intervals of 15 minutes at best even if taken frequently. At a higher frequency, the patient would feel uncomfortable and congestion would occur downstream of the manchette. Also, the manchette has to be wrapped around the upper limb opposite a shunt side, causing the patient to feel uncomfortable with both upper limbs bound.
In this way, since the patient feels uncomfortable with both upper limbs bound if blood pressure is measured from the upper limb on the side opposite the shunt side, it is advisable to measure blood pressure continuously from the upper limb on the shunt side. As a method for measuring blood pressure continuously from the arm on the shunt side during dialysis, Patent Literature 1 describes a method for estimating blood pressure using a shunt blood flow velocity measured with an infrared Doppler meter. The method estimates blood pressure using a relationship (proportionality relationship), measured in advance, between shunt blood flow velocity and blood pressure. With this method, it is necessary to ensure that an infrared beam will hit the center of a flow line of shunt blood flow. This is extremely difficult under circumstances in which the patient will move the arm even slightly. Also, the equipment is expensive. Patent Literatures 2 and 3 disclose methods for estimating blood flow condition at a site of shunt creation by monitoring vibrations and blood flow sounds at the site of shunt creation during dialysis. However, these methods measure a blood flow volume rather than blood pressure. Patent Literature 4 discloses means for measuring pressure in the blood flowing through a shunt blood vessel used during dialysis, identifying and isolating vibrations produced by pumps and the like of a dialysis monitoring system, and thereby measuring a pulse rate and blood pressure values. However, the technique has drawbacks: pressure of drawn blood is low even on the arterial side, it is difficult to isolate noise produced by equipment, there is a possibility that dialyzed blood may flow backward, and so on. Also, the technique has a problem in measurement accuracy.